Anamorphic lenses were originally developed for cinematography in order to provide a wide screen format while making use of a relatively large negative size. For example, the current standard negative size for anamorphic shooting measures 0.825″×0.69″, giving an aspect ratio of 1.1956:1. If a 2× squeeze anamorphic lens is used to form an image on this format, and the image is then de-squeezed by a factor of two then the final aspect ratio is 2.39:1, which is the current DCI Scope ratio used for theatrical projection. If a 2.39:1 aspect ratio were shot directly on a 0.825″ wide format the format size would be 0.825″×0.345″, which is only half the area of the anamorphic format. By using a larger film negative the overall image quality was improved while simultaneously permitting a wide screen ratio, and this provided all the justification needed to develop and use anamorphic optics for cinematography.
As digital cinematography has largely superseded film cinematography, the original reasons for using anamorphic lenses have become less important. However, anamorphic lenses provide unique artistic opportunities due to their residual optical characteristics. Chief among these characteristics for a so-called front anamorphic lens are differential depth of field in the horizontal and vertical directions, along with elliptically-shaped defocused point images oriented so that the short side of the ellipse coincided with the axis of greatest anamorphic power. This last characteristic is also commonly referred to as “elliptical bokeh” or “oval bokeh”, where “bokeh” refers to the character of the defocused image. Bokeh can be thought of as the way a lens renders defocused light points of light and is considered an aesthetic quality that in cinemagraphic applications is considered desirable.
There are two main classes of anamorphic lenses. The first class is the so-called front anamorphic type, in which most or all of the anamorphic power is achieved by means of cylindrical and/or toroidal lens elements placed on the object side of the aperture stop. From now on these cylindrical and/or toroidal lens elements shall be referred to as “anamorphic lens elements”. The second class is the so-called rear anamorphic type, in which most or all of the anamorphic power is achieved by means of anamorphic lens elements placed on the image side of the aperture stop. Generally, the front anamorphic type is the more desirable of the two, since this type produces elliptical bokeh and differential depth of field, but the rear anamorphic type does not when a circular aperture stop is used. Rear anamorphic lenses are typically large zoom lenses, which would be impractically large if designed with the anamorphic power in the front.
In front anamorphic lenses and/or attachments most of the anamorphic optical power is in the horizontal direction. The usual arrangement is an afocal reversed-Galilean anamorphic front which reduces the focal length in the horizontal direction while either leaving the focal length alone in the vertical direction or else modifying it to a lesser extent than in the horizontal direction. It is useful to think of front anamorphics as squeezing the image in the horizontal direction. So, the image of a circular object will be an ellipse with the long direction oriented vertically. The elliptical in-focus image of a circular object is rendered circular during the de-squeezing process.
In rear anamorphic lenses and/or attachments most of the anamorphic optical power is in the vertical direction. The usual arrangement is a tele-extender style rear attachment which increases the focal length in the vertical direction while either leaving the focal length alone in the horizontal direction or else modifying it to a lesser extent than in the vertical direction. In this case it is useful to think of function of the rear anamorph as stretching the image in the vertical direction. The image of a circular object will be an ellipse with the long direction oriented vertically, just as in the case with a front anamorph, and again a de-squeezing process is required in order to transform the elliptical in-focus image of a circular object into a proper circle. However, there are important differences between front and rear anamorphic lenses which will be discussed below.
It is important to note that the desirable residual anamorphic characteristics of differential depth of field and elliptical bokeh do not include the squeeze ratio. The squeeze ratio is essentially a paraxial quantity that is eliminated for display by a de-squeezing process. Thus, an image taken with a 2× squeeze anamorphic on a 4:3 image format will be stretched, or de-squeezed, by the same 2× factor for display. For film display this has historically been done with an anamorphic projection lens, but for digital display this can be easily done during post-processing. After de-squeeze, the elliptical image of a circular object will become properly circular. The desirable anamorphic characteristics mentioned above are what is left over after the de-squeeze process.
Front anamorphic lenses are generally available in a 2× squeeze ratio, which yields the standard 2.39:1 DCI Scope format when shot on a 1.1956:1 aspect ratio film negative or electronic image sensor. Another very useful squeeze ratio is substantially 1.79×, which will yield an exact 2.39:1 DCI Scope format when shot on a 4:3 aspect ratio sensor.
Front anamorphic lenses are also available in approximately 1.3 to 1.35× squeeze ratio Q, which is approximately what is needed to yield Scope format output when shot on a 16:9 aspect ratio image sensor. The actual squeeze ratio Q needed to produce 2.39:1 DCI Scope output from a 16:9 sensor is 1.344×. In other words, if a 1.344× squeeze anamorphic lens is used on a 16:9 sensor and the image is then de-squeezed or stretched by 1.344× the final output will be (16×1.344)/9=2.39:1. A 1.344× squeeze anamorphic is particularly desirable because 16:9 sensors are very common in video and cine cameras. In this case, a 1.344× squeeze anamorphic lens utilizes the full 16:9 sensor without any need to crop the image during post processing.
In addition, if a 1.344× squeeze anamorphic is used on a 4:3 aspect ratio sensor, also common in video and cine cameras, the output aspect ratio after de-squeezing by 1.344× will be 1.792:1, or 16.128:9, which is nearly identical to the standard HDTV aspect ratio of 16:9 which is commonly used for TV broadcast and commercials.
Unfortunately, a front anamorphic lens with a squeeze ratio of 1.344× or smaller will have only mild anamorphic characteristics, and as a result many cinematographers would prefer to simply crop in post rather than bother with the expense of anamorphic. Accordingly, there is a need for anamorphic lenses having a relatively small squeeze ratio Q that retains relatively pronounced anamorphic characteristics of elliptical bokeh and differential depth of field generally found in anamorphic lenses having a relatively large squeeze ratio.
Many anamorphic optical systems have been described in the prior art, but none of these provide for a simultaneous combination of a relatively small squeeze ratio with relatively pronounced anamorphic characteristics. In U.S. Pat. No. 4,362,366 by Gottschalk, a compound anamorphic optical system is disclosed that combines a conventional reversed-Galilean type afocal front anamorphic section and a rear type anamorphic focal length extender. However, in this case the purpose of the rear anamorphic section is to allow a reduced anamorphic power in the front section so that the size and weight of the system as a whole may be reduced while simultaneously permitting improved aberration correction. Accordingly, although the claimed optical system has a large anamorphic squeeze factor it has only mild residual anamorphic characteristics.
In U.S. Pat. No. 8,174,733 by Pretorius and U.S. Pat. No. 8,858,099 by Dodoc et al. anamorphic optical systems are disclosed which have anamorphic lens elements on both sides of the aperture stop. However, the anamorphic lens elements on the image side of the stop in these disclosed designs serves primarily to correct residual aberrations, and similar to the Gottschalk system the result is an anamorphic lens system with a large squeeze factor but only mild residual anamorphic characteristics.